1. Field of the Invention
The present invention relates generally to power converters for providing d.c. rectified power supplies, and, more particularly, to a circuit for limiting the current surge in a storage capacitor at the powering-on of a rectifier.
2. Discussion of the Related Art
FIG. 1 is a schematic block diagram of an exemplary correctional circuit for limiting the current surge in a storage capacity Cf associated with a rectifier 2. The rectifier 2 is connected, through terminals E1 and E2, to an a.c. power supply and provides, on terminals P and M, a d.c. rectified power supply. Typically, terminal M is a ground terminal. Terminals P and M supply a load 1 generally associated with a power converter such as a switched mode power supply. A device for correcting the power factor may be provided, in which case the storage capacitor Cf may be included as part of the device.
The powering-on of the conventional circuit shown in FIG. 1 is generally performed by means of a manually- or automatically-controlled switch 4, located between the a.c. power supply and the rectifier 2. The powering-on of the circuit causes a current surge ranging from several tens to several hundreds of amperes due to the initial charge of capacitor Cf. To avoid this, a device 5 for limiting the current surge is commonly placed between terminal P and capacitor Cf.
Device 5 generally includes a series resistance Rs. In order to avoid the resistor Rs from continuously dissipating power, a switch 6 is generally provided to short-circuit the resistor Rs once the power converter has reached a steady state. The control line 7 of switch 6 is typically provided by the power converter and may be based on, for example, a measurement of the charge current of capacitor Cf.
A disadvantage of a conventional device such as shown in FIG. 1 is that the presence of a series resistance on the rectified power line results in significant power consumption by the resistor Rs during the initial transient charge phase of capacitor Cf.
Another disadvantage of such a conventional device for limiting the current surge upon power-on is that it requires auxiliary circuits that continuously dissipate power. These auxiliary circuits are required to cause and maintain the short-circuiting of resistance Rs in a steady state. For example, if the power converter includes a transformer, the control 7 of switch 6 generally requires an additional secondary winding Further the very presence of switch 6 on the positive rectified power supply line results in steady-state power consumption due to the resistance of switch 6 when it is closed.
Another disadvantage of the device of FIG. 1 is that, should a mains failure occur for a duration such that the voltage across capacitor Cf dissipates below the range associated with steady state, the switch 6 opens, causing the power consumption by resistance Rs.
Another disadvantage is that the operation of the device for limiting the current surge can be altered in case of a temporary drop of the mains voltage. This may occur, for example, after a strong current consumption of another load connected to the electric network. Particularly, when switch 6 is controlled by a relay, and assuming that the voltage drop remains within the devised operating range (that is, switch 6 mans in the "on" state), a very large current peak appears as the mains voltage rises back to the initial level.